This is a rather difficult to understand argument. (Thanks to Dangerous Idea for the link.) Perhaps it can be rephrased in this way:
1. True propositions correspond to reality.
2. “Intentionality is the power of minds to be about, to represent, or to stand for, things, properties and states of affairs.” (Note that Plantinga is confusing correspondence and intentionality. Thanks to Dante Alighieri for this point.)
3. The mind can be about things by thinking propositional statements, e.g., the belief that the cat is on the mat is about the cat, the mat, the state of affairs of the cat being on the mat, etc., while the proposition “the cat is on the mat” corresponds to the same state of affairs.
4. “Representing things as being thus and so, being about something or other — this seems to be a property or activity of minds or perhaps thoughts. So extremely tempting to think of propositions as ontologically dependent upon mental or intellectual activity in such a way that either they just are thoughts, or else at any rate couldn’t exist if not thought of.”; therefore, propositions are mind-dependent; not only that, they are in minds, being thought by them.
5. ∀(x ∈ ℜ)(x is distinct from Taj Mahal).
6. Now expand (5) into an (uncountable) infinity of propositions: 1 is distinct from Taj Mahal; π is distinct from Taj Mahal; etc. All of these are true and therefore exist (wherever). But there are too many of these propositions to be contained in any finite mind.
7. There cannot be, for example, an infinity of “mini-minds,” each one containing a single proposition. This is because there would be an actual infinite of concrete (rather than abstract) things which is impossible. Further, it can be proven by other means that there is only one God.
8. Therefore, propositions must be contained in the infinite mind (of God).
Another argument:
1a. Propositions are necessarily existing abstract objects.
2a. Propositions can exist only in minds or thoughts.
3a. Yet not in human minds, because those are contingent, and there are too many propositions anyway.
4a. Therefore, they must exist in a divine mind.
5a. The existence and necessity of propositions are due to the existence and necessity of God. Or, there is an identity between the “natural unity” (Aquinas’s term) of all propositions or truths in God’s mind and God’s essence, rather like the fact that God is identical with His essence, His existence, Power, Goodness, etc.
6a. Thus, propositions are abstract entities. There are an infinity of them; they exist necessarily, and they exist in a necessarily existing and infinite mind; and altogether, if it can be argued (probably not) that they comprise all knowledge, they are that mind.
I think you’re getting him wrong, I’m just not sure how to reconstruct his argument better than you did. Note that this is the argument from intentionality and your premises take no note of that.
I think what you miss, is the quote from Aquinas:
(Aquinas, De Veritate “Even if there were no human intellects, there could be truths because of their relation to the divine intellect. But if, per impossibile, there were no intellects at all, but things continued to exist, then there would be no such reality as truth.”)
I think Plantinga is linking truth and propositions+intentionality here. So Truth is a two-sided affair. Only propositions are true. They are true in virtue of representing things as they actually are. So if we suppose propositions come into being with the thinking of them, then if God did not exist, there would be a time when nothing was true. But it seems absurd to say that there was a time when nothing was true.